Metal halide lamp having ionizable iodide salt

ABSTRACT

A metal halide lamp comprising a substantially cylindrical discharge vessel having an internal diameter Di&lt;2.0 mm and filled with an ionizable filling, wherein two electrodes are present at a mutual distance EA for maintaining a discharge in the discharge vessel, wherein the filling comprises an inert gas, such as Xe, having a pressure at room temperature between 5 and 25 bar, and an ionizable salt, characterized in that said ionizable salt is selected from the group comprising PrI3, NdI3 and LuI3.

The invention relates to an automotive metal halide lamp comprising asubstantially cylindrical discharge vessel having an internal diameterDi<2.0 mm, and filled with an ionizable filling, wherein two electrodesare present at a mutual distance EA, wherein preferably 3 mm<EA<7 mm,for maintaining a discharge in the discharge vessel, and wherein thefilling comprises an inert gas such as Xe having a pressure at roomtemperature between 5 and 25 bar, and an ionizable salt.

Such a lamp is described in the international patent application WO00/67294. Many modern automotive metal halide lamps have a very small,very high pressure discharge vessel surrounded by a gas-filled outerbulb, and have a lamp power between 20 W and 40 W. The filling of thelamp can contain Hg, or alternatively can be mercury-free and contain Znor ZnI₂. Such lamps require highly efficient ionizable salts, and it isknown to use a salt mixture of NaI and CeI₃. Such a lamp is based on therecognition that a high efficacy and a sufficiently high color renderingis possible when sodium halide is used as a filling ingredient of a lampand a strong widening and inversion of the Na emission in the Na-D linestakes place during lamp operation. This requires a high coldest-spottemperature in the discharge vessel, which excludes under practicalconditions the use of quartz or quartz glass for the discharge vesselwall and renders the use of a ceramic material for the discharge vesselwall preferable. The term “ceramic wall” in the present description andclaims is understood to cover a wall of metal oxide such as, forexample, sapphire or densely sintered polycrystalline Al₂O₃, as well asmetal nitride, for example AlN. The known lamp combines a good colorrendering with a comparatively wide range of the color temperature.

The lamp has the advantage that the discharge vessel has very compactdimensions which render the lamp highly suitable for use in a headlampfor a motor vehicle. Owing to the small internal diameter in comparisonwith the electrode spacing, and thus the discharge arc length, thedischarge arc is hemmed in by the discharge vessel wall, so that thedischarge arc has a sufficiently straight shape for it to be suitablefor use as a light source for a motor vehicle headlamp. A small internaldiameter Di is found to be of essential importance for realizing a sharpbeam delineation necessary for use in motor vehicles in combination witha small spot of high brightness immediately adjacent this delineation.Such very small internal diameter renders the lamp particularly suitablefor use as a light source in a complex-shape headlamp. An advantage ofsuch a headlamp is that no separate passing-beam cap is required in theformation of the light beam to be generated in order to realize asufficiently sharp beam delineation.

The drawbacks of the known lamp are however a relatively low correlatedcolor temperature (CCT) (between 3000 and 3500 K), a relatively unstableluminous flux, a relatively unstable wall temperature, a relativelylarge initial color point spread and a relatively large color pointshift during life time, mainly due to chemical transport and segregationof the NaI/CeI₃ salt mix.

The object of the invention is an automotive metal halide lamp whereinone or more of the above-mentioned drawbacks are alleviated. In order toachieve that goal, said ionizable salt is selected from the groupcomprising PrI₃, NdI₃ and LuI₃. Preferably said ionizable salt furthercomprises NaI, wherein the molar ratio NaI/(PrI₃+NdI₃+LuI₃) lies between1.0 and 10.3. Although usually one of the above-mentioned rare earthiodides will be used, it is possible to use a mixture as well. It wasfound that in a lamp of the above-mentioned properties these salts areonly slightly sensitive to big variations in lamp power and thus incoldest spot temperature, while showing a color spot close to the BBL(“black body line”), and that these salts are relatively insensitive tocolor shifts due to segregation, i.e. changes in the salt mix ratio atthe coldest spot position of the lamp due to for instance corrosion ortransport of the liquid salt. In particular the use of PrI₃ results inan excellent color temperature for automotive purposes, close to thepreferred CCT of 4200 K, while if LuI₃ is used for instance the colortemperature can be enhanced by adding small amounts of TbI₃ and/or GdI₃.

In a first preferred embodiment the molar ratio NaI/PrI₃ lies between2.3 and 10.3, preferably between 3.0 and 5.7, and more preferably isapproximately 3.5. Preferably the amount of PrI₃ in the discharge vesselis between 10 and 335 μmol/cm³, more preferably between 25 and 160μmol/cm³, still more preferably approximately 50 μmol/cm³. In adischarge vessel of 1.6 mm×8 mm (Di×EA) this results in a CCT ofapproximately 4200 K. In a discharge vessel of 1.2 mm×6 mm the preferredconcentration is 1.8 times higher in order to have the same CCT.

In a second preferred embodiment the molar ratio NaI/NdI₃ lies between 3and 6.7, preferably between 3.6 and 4.8, and more preferably isapproximately 4.2. Preferably the amount of NdI₃ in the discharge vesselis between 8 and 301 μmol/cm³, more preferably between 30 and 167μmol/cm³, still more preferably approximately 45 μmol/cm³. In adischarge vessel of 1.6 mm×8 mm (Di×EA) this results in a CCT ofapproximately 4200 K. In a discharge vessel of 1.2 mm×6 mm the preferredconcentration is 1.8 times higher in order to have the same CCT.

In a third preferred embodiment the molar ratio NaI/LuI₃ lies between1.0 and 3.2, preferably between 1.2 and 1.8, and more preferably isapproximately 1.4. Preferably the amount of LuI₃ in the discharge vesselis between 15 and 414 μmol/cm³, more preferably between 27 and 230μmol/cm³, still more preferably approximately 69 μmol/cm³. In adischarge vessel of 1.6 mm×8 mm (Di×EA) this results in a CCT ofapproximately 4200 K. In a discharge vessel of 1.2 mm×6 mm the preferredconcentration is 1.8 times higher in order to have the same CCT.

These and other aspects of the lamp according to the invention will beexplained in more detail with reference to the drawings (not to scale),wherein:

FIG. 1 diagrammatically shows a lamp according to the invention; and

FIG. 2 shows the discharge vessel of the lamp of FIG. 1 in detail.

FIG. 1 shows a metal halide lamp provided with a discharge vessel 3having a ceramic wall which encloses a discharge space 11 containing anionizable filling. Two tungsten electrodes 4, 5 whose tips 4 b, 5 b areat a mutual distance EA are arranged in the discharge space, and thedischarge vessel has an internal diameter Di at least over the distanceEA. The discharge vessel is closed at one side by means of a ceramicprojecting plug 34, 35 which encloses a current lead-through conductor(FIG. 2: 40, 41, 50, 51) to an electrode 4, 5 positioned in thedischarge vessel with a narrow intervening space and is connected tothis conductor in a gastight manner by means of a melting-ceramic joint(FIG. 2: 10) at an end remote from the discharge space. The dischargevessel is surrounded by an outer bulb 1 which is provided with a lampcap 2 at one end. A discharge will extend between the electrodes 4, 5when the lamp is operating. The electrode 4 is connected to a firstelectrical contact forming part of the lamp cap 2 via a currentconductor 8. The electrode 5 is connected to a second electrical contactforming part of the lamp cap 2 via a current conductor 9. The dischargevessel, shown in more detail in FIG. 2 (not to scale), has a ceramicwall and is formed from a cylindrical part with an internal diameter Diwhich is bounded at either end by a respective ceramic projecting plug34, 35 which is fastened in a gastight manner in the cylindrical part bymeans of a sintered joint S. The ceramic projecting plugs 34, 35 eachnarrowly enclose a current lead-through conductor 40, 41, 50, 51 of arelevant electrode 4, 5 having a tip 4 b, 5 b. The current lead-throughconductor is connected to the ceramic projecting plug 34, 35 in agastight manner by means of a melting-ceramic joint 10 at the sideremote from the discharge space. The electrode tips 4 b, 5 b arearranged at a mutual distance EA. The current lead-through conductorseach comprise a halide-resistant portion 41, 51, for example in the formof a Mo—Al₂O₃ cermet and a portion 40, 50 which is fastened to arespective end plug 34, 35 in a gas tight manner by means of themelting-ceramic joint 10. The melting-ceramic joint extends over somedistance, for example approximately 1 mm, over the Mo cermet 40, 41. Itis possible for the parts 41, 51 to be formed in an alternative mannerinstead of from a Mo—Al²O³ cermet. Other possible constructions areknown, for example, from EP 0 587 238. A particularly suitableconstruction was found to be a halide-resistant coil applied around apin of the same material. Mo is very suitable for use as a highlyhalide-resistant material. The parts 40, 50 are made from a metal whosecoefficient of expansion corresponds very well to that of the end plugs.Nb, for example, is a highly suitable material for this purpose. Theparts 40, 50 are connected to the current conductors 8, 9 in a mannernot shown in any detail. The lead-through construction described rendersit possible to operate the lamp in any desired burning position. Each ofthe electrodes 4, 5 comprises an electrode rod 4 a, 5 a which isprovided with a tip 4 b, 5 b.

In a practical embodiment of the lamp as represented in the drawing, anumber of lamps were manufactured with a rated power of 26 W each. Thelamps are suitable for use as headlamps for a motor vehicle. Theionizable filling of the discharge vessel 3 of each individual lampcomprises 30 mg/cm³ Hg and 25 mg/cm³ iodide, comprising NaI and a rareearth iodide chosen from the group consisting of PrI₃, NdI₃ and LuI₃. Ina mercury-free embodiment Hg may be replaced by Zn or ZnI₂. The fillingfurther comprises Xe with a filling pressure at room temperature of 8bar. The distance between the electrode tips 4 a, 5 a EA is 5 mm, theinternal diameter Di is 1.4 mm, so that the ratio EA/Di=3.6. The wallthickness of the discharge vessel 3 is 0.3 mm.

In a first embodiment the rare earth iodide is PrI₃ at approximately 50μmol/cm³, and the molar ratio NaI/PrI₃ is approximately 3.5.

In a second embodiment the rare earth iodide is NdI₃ at 45 μmol/cm³, andthe molar ratio NaI/NdI₃ is approximately 4.2.

In a third embodiment the rare earth iodide is LuI₃ at 69 μmol/cm³, andthe molar ratio NaI/LuI₃ is approximately 1.4. In order to improve thecolor temperature of this lamp, small amounts of TbI₃ or GdI₃ wereadded.

The lamps described showed excellent color temperature and colorstability properties compared to NaI/CeI₃ fillings, while the efficacyis only slightly lower.

1. A metal halide lamp comprising a substantially cylindrical discharge vessel (3) having an internal diameter Di<2.0 mm and filled with an ionizable filling, wherein two electrodes are present at a mutual distance EA for maintaining a discharge in the discharge vessel, wherein the filling comprises an inert gas having a pressure at room temperature between 5 and 25 bar, and an ionizable salt, characterized in that said ionizable salt is selected from the group comprising PrI₃, NdI₃ and LuI₃.
 2. A lamp according to claim 1, wherein said ionizable salt further comprises NaI, and wherein the molar ratio NaI/(PrI₃+NdI₃+LuI₃) lies between 1.0 and 10.3.
 3. A lamp according to claim 2, wherein the molar ratio NaI/PrI₃ lies between 2.3 and 10.3.
 4. A lamp according to claim 3, wherein the molar ratio NaI/PrI₃ lies between, preferably between 3.0 and 5.7.
 5. A lamp according to claim 4, wherein the molar ratio NaI/PrI₃ is approximately 3.5.
 6. A lamp according to claim 2, wherein the molar ratio NaI/NdI₃ lies between 3.0 and
 67. 7. A lamp according to claim 6, wherein the molar ratio NaI/NdI₃ lies between 3.6 and 4.8.
 8. A lamp according to claim 7, wherein the molar ratio NaI/NdI₃ is approximately 4.2.
 9. A lamp according to claim 2, wherein the molar ratio NaI/LuI₃ lies between 1.0 and 3.2.
 10. A lamp according to claim 9, wherein the molar ratio NaI/LuI₃ lies between 1.2 and 1.8.
 11. A lamp according to claim 10, wherein the molar ratio NaI/LuI₃ is approximately 1.4.
 12. A lamp according to claim 1, wherein the amount of PrI₃ in the discharge vessel is between 10 and 335 μmol/cm³.
 13. A lamp according to claim 12, wherein the amount of PrI₃ in the discharge vessel is between 25 and 160 μmol/cm³.
 14. A lamp according to claim 13, wherein the amount of PrI₃ in the discharge vessel is approximately 50 μmol/cm³.
 15. A lamp according to claim 1, wherein the amount of NdI₃ in the discharge vessel is between 8 and 301 μmol/cm³.
 16. A lamp according to claim 15, wherein the amount of NdI₃ in the discharge vessel is between 30 and 167 μmol/cm³.
 17. A lamp according to claim 16, wherein the amount of NdI₃ in the discharge vessel is approximately 45 μmol/cm³.
 18. A lamp according to claim 1, wherein the amount of LuI₃ in the discharge vessel is between 15 and 414 μmol/cm³.
 19. A lamp according to claim 18, wherein the amount of LuI₃ in the discharge vessel is between 27 and 230 μmol/cm³.
 20. A lamp according to claim 19, wherein the amount of LuI₃ in the discharge vessel is approximately 69 μmol/cm³.
 21. A lamp according to claim 1, wherein Di<1.5 mm.
 22. A lamp according to claim 1, wherein EA lies between 3 mm and 7 mm.
 23. A lamp according to claim 1, wherein the discharge vessel has a ceramic wall.
 24. A lamp according to claim 1, wherein the discharge vessel is surrounded by a gas-filled outer bulb.
 25. A lamp according to claim 1, wherein the lamp power lies between 20 W and 40 W.
 26. A metal halide lamp comprising a substantially cylindrical discharge vessel (3) having an internal diameter Di<2.0 mm and filled with an ionizable filling, wherein two electrodes are present at a mutual distance EA for maintaining a discharge in the discharge vessel, wherein the filling comprises an inert gas having a pressure at room temperature between 5 and 25 bar, and an ionizable salt selected from the group comprising PrI₃, NdI₃ and LuI₃, wherein the amount of NdI₃ in the discharge vessel is between 8 and 301 μmol/cm³.
 27. A lamp according to claim 26, wherein the amount of NdI₃ in the discharge vessel is between 30 and 167 μmol/cm³.
 28. A lamp according to claim 27, wherein the amount of NdI₃ in the discharge vessel is approximately 45 μmol/cm³.
 29. A metal halide lamp comprising a substantially cylindrical discharge vessel (3) having an internal diameter Di<2.0 mm and filled with an ionizable filling, wherein two electrodes are present at a mutual distance LA for maintaining a discharge in the discharge vessel, wherein the filling comprises an inert gas having a pressure at room temperature between 5 and 25 bar, and an ionizable salt selected from the group comprising PrI₃, NdI₃ and LuI₃, wherein said ionizable salt further comprises NaI, and wherein the molar ratio NaI/(PrI₃+NdI₃+LuI₃) lies between 1.0 and 3.2.
 30. A lamp according to claim 29, wherein the molar ratio NaI/(PrI₃+NdI₃+LuI₃) lies between 1.2 and 1.8.
 31. A lamp according to claim 30, wherein the molar ratio NaI/(PrI₃+NdI₃+LuI₃) is approximately 1.4.
 32. A metal halide lamp comprising a substantially cylindrical discharge vessel (3) having an internal diameter Di<2.0 mm and filled with an ionizable filling, wherein two electrodes are present at a mutual distance EA for maintaining a discharge in the discharge vessel, wherein the filling comprises an inert gas having a pressure at room temperature between 5 and 25 bar, and an ionizable salt selected from the group comprising PrI₃, NdI₃ and LuI₃, wherein the amount of LuI₃ in the discharge vessel is between 15 and 414 μmol/cm³.
 33. A lamp according to claim 32, wherein the amount of LuI₃ in the discharge vessel is between 27 and 230 μmol/cm³.
 34. A lamp according to claim 33, wherein the amount of LuI₃in the discharge vessel is approximately 69 μmol/cm³. 